Astrophile: Blobby old galaxy boasts hidden arms

Object&colon; The giant elliptical galaxy Centaurus ADistance&colon; About 12 million light years

Centaurus A was facing a midlife crisis. The giant elliptical galaxy’s brightest stars were old and puffy, and it had nearly run out of gas needed to create new ones. The galaxy was just a featureless blob that had lost its sparkle.

Then a chance encounter allowed boring old Centaurus A to have a fling with a younger, smaller galaxy. The event revived the elder partner, triggering a fresh round of star birth and creating one of its most notable features&colon; a dark dust lane along its middle.

In a surprise twist, new observations show the cosmic hanky-panky also caused Centaurus A to sprout two spiral arms – something no other elliptical galaxy is known to have. The discovery offers new insights into how galaxies form and evolve, and hints at a new way for spiral structure to emerge.

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Shocking revelation

The bisecting dust lane led astronomers in the early 19th century to think that Centaurus A might be two separate objects lying side by side. More recent studies have shown that the dust is most likely a disc left behind by a galactic merger.

By blocking visible light, the dust also conceals the intimacies of the galaxy’s steamy affair. To gather more clues, Daniel Espada of the National Astronomical Observatory of Japan and colleagues looked at Centaurus A in radio wavelengths.

These longer waves emerge from carbon monoxide gas at the galaxy’s centre and can pierce the dusty veil, allowing the team to trace otherwise hidden structures. What they saw was shocking.

“We were quite surprised to find what clearly looked like spiral arms,” says team member Alison Peck of the Joint ALMA Observatory in Santiago, Chile. Their images show the tentacles of gas curving around the galaxy’s middle, with widths and orientations similar to those of the arms of spiral galaxies like our Milky Way.

What’s more, the gas tentacles are “moving in a way that you would expect spiral arms to move”, says Peck.

Gassy tendrils

Bruce Elmegreen, a spiral-structure expert at the IBM Research Division in Yorktown Heights, New York, says the arms on Centaurus A are unique because they are made of molecular gas instead of stars, the main components of the Milky Way’s arms.

“It’s really unusual,” says Elmegreen. Usually a disc of gas around an object rotates in such a way that the gas clumps up rather than forming long filaments.

He suspects that Centaurus A is so massive and its stars are so centrally concentrated that the galaxy’s rotating core creates a huge shear effect, one that sculpted the gas into spiral structures.

Since galaxies grow via mergers, the new study suggests it is possible that spiral arms exist on other elliptical galaxies. Unfortunately, most are too far away for us to get a detailed peek.